Water supply networks have to be designed while taking into account the long-term future water demand. Given the difficulties in estimating the future water demand, a flexible approach for planning water supply systems was implemented. The flexibility in engineering design approach (de Neufville and Scholtes, 2011) is a procedure for designing systems in the face of an uncertain future. This approach was applied to a fictitious town, which is expected to expand in the near future and will need to extend its water supply network to the new areas.
To model the uncertain future water demand, 1000 scenarios were generated by randomly projecting the population of the town. A decision rule, which governs when the network must be modified, was defined based on the pressure that is required at the nodes of the network. Four different design alternatives were tested: the deterministic design, which is completely built in advance based on one single scenario, and three flexible designs, which consist in expanding the network as the water demand changes.
The design alternatives were simulated using the hydraulic solver EPANET and were evaluated in terms of costs, using the specific net present value, and in terms of performance based on the pressure and residence time of water in the network. The results showed that the flexible design alternatives provide a significant decrease in the specific costs compared to the deterministic design, thus demonstrating the value of flexible designs. By evaluating the designs over a range of water demand scenarios, different results were obtained compared to the case where only one scenario was used. This showed that the flexibility in engineering design approach is a helpful tool for designing water supply networks in the context of an uncertain future.